Low-pressure mercury vapor lamps, more commonly known as fluorescent lamps, have a lamp envelope with a filling of mercury and rare gas to maintain a gas discharge during operation. The radiation emitted by the gas discharge is mostly in the ultraviolet (UV) region of the spectrum, with only a small portion in the visible spectrum. The inner surface of the lamp envelope has a luminescent coating, often a blend of phosphors, which emits visible light when impinged by the ultraviolet radiation.
There is an increase in the use of fluorescent lamps because of reduced consumption of electricity. To further reduce electricity consumption, there is a drive to increase efficiency of fluorescent lamps, referred to as luminous efficacy which is a measure of the useful light output in relation to the energy input to the lamp, in lumens per watt (LPW).
U.S. Pat. No. 5,552,665 Of Charles Trushell, an inventor in the present application, relates to an electric lamp having a luminescent layer on the lamp envelope which produces visible light when impinged by ultraviolet radiation generated within the lamp, and wherein an undercoat for the luminescent layer is employed. The disclosure of said patent is hereby incorporated by this reference thereto. Such an undercoat is now a common feature of modern fluorescent lamps, and is an oxidic, particulate base coat layer of non-fluorescent material, preferably an aluminum oxide, underlying the light-giving phosphor. Such an undercoat or base-coat is intended to economically increase light output, simplify the manufacturing process, improve the maintenance of light output, and reduce mercury consumption by the glass bulb. Typically, such layers are composed of very small particles with consequently large surface areas. Unfortunately, it has been found that the large surface of the particulate base-coat combined with the propensity of aluminum oxide to adsorb gaseous molecules results in larger than normal amounts of contaminants being introduced into the lamp interior during manufacture. For example, water and carbon dioxide are common, volatile, fluorescent lamp contaminants, the amounts of which are increased as a result of the large surface area of the undercoat. One effect of the increased amount of these contaminants is to increase the duration of arc instability immediately after lamp ignition.
It is known to coat the phosphor layer contained in a fluorescent lamp. For example:
Tamura, Japanese Patent Application No. 03179238 (Abstract)), describes a procedure wherein MgO is mixed with a phosphor at 0.01-1.0% and used to form a layer as a step in the manufacture of a fluorescent lamp in order to getter CO2 and CO impurities which exist after the lamp is manufactured.
Watanabe et al, U.S. Pat. No. 5,604,396, describes a method wherein an alcoholic solution of a metal alkoxide (wherein the metal may be any of numerous metals including magnesium) is added to an aqueous suspension of a phosphor, which is to be coated by the alkoxide. Upon evaporation of the alcohol, the alkoxide is converted to the hydroxide and homogeneously precipitated on the surface of the phosphor in a sol-gel process. After removal of the water, the hydroxide-coated phosphor is fired at a high temperature; however, no specific benefits are claimed for coating the phosphor with the metal alkoxide. Moreover, we have found that coating the phosphor with metal alkoxide or metal oxide does not eliminate or mitigate the increase in duration of the arc instability in the lamp when an oxidic base-coat such as alumina is used.
There is a need in the art for a means of reducing the amount of contaminants and for eliminating or at least mitigating the increase in duration of arc instability to which the contaminants contribute in a fluorescent lamp.